Function interp_exists 

fn interp_exists(
    positional: &[RValue],
    _named: &[(String, RValue)],
    context: &BuiltinContext<'_>,
) -> Result<RValue, RError>

Test whether a variable exists in an environment.

@param x character string giving the variable name @param envir environment to search in (default: calling environment) @return TRUE if the variable exists, FALSE otherwise